The invention relates to the genetic manipulation of plants, particularly to altering sulfur metabolism in plants and plant seeds.
Sulfur in its reduced form plays an important role in plant metabolism, being involved in the biosynthesis of a wide range of primary and secondary S-containing metabolites. In plants, sulfur metabolism includes the uptake of sulfate from the environment, assimilation into organic compounds, and channeling into proteins and secondary substances.
Plants and microorganisms are able to reduce sulfate to sulfide for synthesis of the thiol group of cysteine. Sulfate is first activated by ATP sulfurylase, forming 5xe2x80x2-adenylylsulfate (APS). APS can be phosphorylated by APS kinase, forming 3xe2x80x2-phosphoadenosine-5xe2x80x2-phosphosulfate (PAPS). Either APS or PAPS can be used for sulfate reduction. Generally, prokaryotes and fungi use PAPS, whereas photosynthetic eukaryotes use APS.
Cysteine, methionine, and sulfur-containing vitamins such as biotin or thiamine are essential in human nutrition. Sulfur-mediated functions include electron transport in Fe/S-clusters, structural and regulatory roles via protein disulfide bridges, and catalytic centers. Additionally, secondary sulfur compounds include signaling molecules, anti-carcinogens and atmospheric compounds. See Hell (1997) Planta 202:138.
Often plant protein is deficient in the sulfur amino acids, especially methionine, as well as other essential amino acids such as lysine and tryptophan. As a result, diets must be supplemented with these amino acids in order to provide a balanced diet. A goal of plant breeding has been to increase the amount of sulfur amino acids present in the seed.
A number of methods have been described for increasing sulfur amino acid content of plants. Generally, these methods provide for the overexpression of a high methionine seed storage protein. The method entails overexpressing the seed storage protein in a transformed plant. Previously, methods for increasing the sulfur amino acid content of crops were attempted through breeding. However, these methods have met with limited success. There is therefore a need for methods of producing significant levels of the sulfur amino acids in plants and plant seeds.
Aerobic organisms are vulnerable to damage from reactive oxygen species. This is a particular problem for plants because reactive oxygen species are generated as a byproduct of oxygenic photosynthesis and carbon dioxide fixation. It would therefore be desirable to provide a method for reducing oxidative stress in plants and for increasing the nutritional quality of plants and seeds.
An object of the present invention is to provide methods for increasing the nutritional value of plants.
Another object of the present invention is to provide plants and plant parts having increased nutritional value.
Another object of the present invention is to provide plants and plant parts having increased levels of organic sulfur compounds.
Another object of the present invention is to provide plants and plant parts having increased levels of methionine.
Another object of the present invention is to provide a method for decreasing oxidative stress in plants.
In accordance with the present invention, methods for modulating the level of at least one organic sulfur compound in plants are provided. Also provided are plants, plant tissues, plant seeds and plant cells produced by the methods. The methods comprise stably transforming a plant with a DNA construct encoding a (P)APS reductase enzyme. (P)APS reductase is defined as an enzyme that is capable of reducing sulfur in the form of APS or PAPS to produce sulfite. The (P)APS reductase) enzyme has an activity such that, the transformed plant exhibits altered levels of at least one organic sulfur compound. Also provided is a method for reducing oxidative stress in plants.